Powder rolling of tungsten and its alloys



April 12, 1966 'r. J. READY ETAL POWDER ROLLING OF TUNGSTEN AND ITS ALLOYS Filed April 24, 1964 INVENTOR.

Homer 0. Lewis John E, Hacker) Thomas .1. Ready,

United States Patent 3,245,114 POWDER ROLLING 0F TUNGSTEN AND ITS ALLOYS Thomas J. Ready and Homer 1). Lewis, Los Alamos, and

John E. Hoclrett, Espanola, N. Mex, assignors to the United States of America as represented by the United States Atomic Energy Commission- Filed Apr. 24, 1964, Ser. No. 362,538 3 Claims. (Cl. 189) This invention relates to a device for compressing metal powders into shapes such as strips. More particularly, the invention relates to an improved powder rolling device which permits satisfactory rolling of powders of tungsten and its alloys.

In powder rolling, powder is compressed into strips by introducing the powder into a roll gap formed by laterally spaced, oppositely disposed pressure rolls. As the powder passes through the roll gap, it is subjected to suificient pressure to cause the powder to cohere and form a strip. The roll gap may be varied to provide the desired thickness of strip. After powder rolling the resulting article is subjected to a sintering treatment in order to increase the strength of the compacted article. Thereafter the article is further rolled, either hot or cold, to effect an increase in density and strength.

The greatly increased use of high temperature applications has required the increased implementation of refractory metals. Since sheets or plates of refractory metals and their alloys are often utilized in these applications (e.g., nuclear reactors), powder rolling is of interest in fabricating the metals. It has been found to be very difficult to powder roll refractory metals and their alloys. Tungsten and its alloys are perhaps the hardest to satisfactorily and reproducibly roll. Tungsten is very britttle and not ductile; consequently there is no tendency to pressure weld during powder rolling. Furthermore, the tungsten powder is normally supplied in very small particle size (e.g., 1 to about 20 micron average particle size), whereas it is known from the prior art that it is very difficult to powder roll materials of less than 44 micron particle size. For example, in attempting to powder roll tungsten and mixtures of WUO it was found that even good green powder-rolled strip had 15% variation in thickness and density. In addition, many of the strips were characterized by edge cracks, laminations and often, a center line crack. Another major problem was the inability to powder roll a newly supplied lot of tungsten powder which was supposedly identical to a lot which had been successfully powder rolled under identical conditions.

The present invention comprises powder rolling apparatus so modified as to permit the successful and reproducible powder rolling of tungsten and its alloys. It should, of course, be understood that the device may also be substituted for more conventional apparatus under any conditions, i.e., the powder rolling of more easily rolled materials.

This unique utility (being capable of rolling tungsten, its alloys or more easily rolled materials) is obtained with a combination comprising a hopper, said hopper including bafile means, a rectangular metering orifice, an orifice width adjustment plate anda pivoted lip at the other side of said orifice, the lip of said hopper being vibrated by electronic driver means, two rollers, at least one of said rollers being stepped, side retention plates placed in the recess caused by said step, the stepped surface being constantly roughened by abrasive 'means while rotating.

An object of the present invention is to provide a refined apparatus for powder rolling.

Another object of this invention is to provide a powder rolling apparatus which will insure satisfactory and reproducible powder rolling.

A further object of this invention is to provide a powder rolling apparatus which will permit the satisfactory and reproducible rolling of tungsten and its alloys.

For a more detailed description of the invention, ref erence may be had to drawing in which:

The device will be described in connection with the rolling of a W-45 v./o. U0 green strip. It should be understood, however, that this is merely one embodiment and that the apparatus of the present invention is not limited thereto.

Three steps were performed in preparing the tungsten and U0 for powder rolling. The two components are blended, the mixing time being arbitrarily selected as two hours. Although no systematic study of uniformity of the dispersion of the U0 in the tungsten was made, examination of many microsections indicated no gross segregation of agglomeration of the U0 The uniformity of the dispersion of U0 was considered very good.

One of the many variables which was believed to be affecting the flow characteristics as well as the powder rolling characteristics of the dispersion was the surface oxide film on the metal powder particles. In order to minimize the effect of the oxide film as a variable, all powders used in this investigation were hydrogen reduced at 700 C. for one hour. This second step simultaneously reduced the uranium oxide to U0 The reduced dispersions were stored under a positive hydrogen atmosphere until they were used.

In the third step, all powders were humidified in a constant humidity atmosphere for four hours immediately prior to powder rolling. The humidity level was selected at 35% relative since it seems that there is a humidity range in which both flow and rolling characteristics of fine metal powder and metal-dispersoid mixtures is optimum.

The material to be powder rolled is stored in the hopper 1. A constant powder head above the metering orifice 2 is necessary for uniform flow rate to the rolls 3. This constant powder head is achieved by inserting a series of bafiles 4,in the hopper, Powder is poured into the hopper and falls by gravity between the baffle openings. As powder is discharged, it is continuously replaced above the orifice and takes on a constant height. Without a constant head, the flow rate would change as the powder head decreases. A minimum of four bafiles seems to be required in order to obtain a sufficiently constant head when powder rolling tungsten and its alloys. Less can be used when rolling less difficult materials.

The orifice adjustment slide 5 controls the size of the metering orifice 2; movement of the orifice adjustment slide being controlled by adjusting the micrometer heads 6. The slide, which is the only moving part of the hop per, is held firmly against the adjusting micrometer heads 6 by a stiff spring 7. There is about 0.005 inch clearance between each side of the slide and the hopper. This allows minor adjustment of the slide to produce uniform flow from the hopper. The actual size of the ori fice may be measured by inserting feeler gage stock between the metering edges. Vibration of the fixed metering edge was accomplished by mechanically linking an audio driver 8 to the lip of the chute face. The audio driver was driven by the amplified signal from a variable frequency oscillator. Amplitude of vibration at the discharge orifice 2 was measured at the back side of the vibrating face at three symmetrically spaced points. The amplitude (e.g., 0.004) and frequency (e.g., 200-400 c.p.s.) of vibration at the three points was always found to be the same. The use of an audio driver with its wide range of amplitude and frequency was found to be effective whereas other methods of vibration (e.g. hammers) were found to be unsatisfactory for tungsten and its alloys. The uniformity and reproducibility of flow and flow rate of the powder through the orifice was measured 'by inserting three weighing boxes below the metering orifice.

In the specific example, all powder rolling was conducted with 8" x 8" rolls, the roll drive had a 15 to 30 feet per minute constant power range and a 2 to 15 feet per minute constant torque range. The roll was operated at a roll speed of 2 /2 feet per minute.

The powder rolling of tungsten or W-UO requires precise control of several vairables before repeated success may be attained. One hundred percent retention of powder in the rolled bite is accomplished by machining a step 9 in the adjustable roll which was greater in depth than the projected roll opening plus the take up of the rolls. By machining the side retainers 10 to just fit the corresponding roll diameters, no powder is lost.

The hopper used for this work fed a 4" wide powder stream to the rolls. With the erratic nonuniform feed at the edges, it was necessary to eliminate the outer A" of each side of the stream and roll a strip only 3 /2 wide. This was accomplished by installing knife edges 11 on the side retainer in the same vertical plane as the side retainers. At first it was believed that a slight excess of powder at the edges was desirable; consequently a sloping A" step was cut into the side retainers. However this excess, in conjunction with the 100% retention, appeared to cause more side defects in the strip and these could only be eliminated by precise metering. The precise metering and 100% retention eliminated all edge failure in the strip and a 3" wide usable strip could be reproducibly rolled.

An important contribution to the success of the device is the control of friction between the rolls and the green strip. The friction is increased by a simple roughening device 12 (in the preferred embodiment, 240-grit silicon carbide paper held firmly against the rolls). Finer grits were tried, but failed to increase friction sufiiciently to eliminate the center line splitting of strip. By successively increasing the friction, better and better strip was produced until strips without defects could be repeatedly rolled with the use of 240-grit paper. The roughness produced by the 240-grit paper was measured to be about 90 microinches (i.e., the ridges were 90 microinches above the valleys). Even with very thin strips (e.g., 0.030") the grooves only amount to about three-tenths of one percent of the as-rolled thickness. Grooves larger than this are detrimental to the rolled product in that stress concentrations result in splitting and cracking. Grooved rollers have been previously used but these were relatively large grooves (e.g., -10% of the as-rolled thickness), of a size which would result in splitting and cracking of tungsten and its alloys. Functionally these large grooves were utilized for the purpose of eliminating low density edges. The function of the grooves in the present device is to provide sufiicient friction force between roll surfaces and powder strip to negate the transverse tensile forces caused by rolling. In addition, the rolls of the present device are continuously roughened to produce a constant coeflicient of friction between the rollers and strip, whereas the rolls of the prior art device were roughened only once before use. It should be noted that although FIGURE 1 shows both rollers stepped and only one roller roughened this has been done only to more clearly portray the device. Both rollers are constantly roughened but only one need be stepped (although both can be stepped).

The original concept of powder rolling of W and W- UO suggested that the height and uniformity of powder in the roll bite would control the quality and uniformity of powder rolled strip. The hopper was to be used as the primary control of feed rate, being adjusted so that the amount per unit time, of powder fed to the roll bite was just equal to the amount, per unit time, of the strip being rolled. However, the design of the hopper could not be refined to this extent. It was then decided to use the hopper for supplying a uniform feed to the rolls and to control the height of powder in the roll bite manually. One of the side retainers was scribed at A1" intervals and referred to the point of contact of the rolls. Visual observation of the height of powder was referred to the scribed marks and a manual off-on switch between the signal generator and amplifier permitted control of the height of powder in the roll nip to i%". The lack of vibration at the metering orifice promptly allows a powder bridge to form, thereby stopping the flow of powder to the roll gap. This proved to be adequate control for the powder rolling of uniform strip. Clearly this control could be effected by automatic means rather than the manual operation described above.

Sulficient strips of W-45 v./o. U0 and unalloyed W have been powder rolled to demonstrate the feasibility of this procedure for such materials. Typical powder rolling conditions for either W45 v./o. U0 or unalloyed W were: roll opening 0.008"; height of powder in roll bite 2 and roll speed 2 /2 feet per minute. The average thickness of the as-rolled strip was 0.057" with a uniformity of :4%. Density data taken indicated a density around of theoretical (15.57 g./cc.) with a variation of :3%. The green strip is subsequently sintered and hot rolled to form the finished sheet.

The dispersion alloy appears to be capable of supporting up to of the stress that can be supported by tensile bars machined from commerical tungsten rods.

All of the described apparatus is necessary in order that tungsten or its alloys be satisfactorily and reproducibly rolled but it will be apparent that many parameters (e.g., roll size, speed, etc.) may be varied without departing from the spirit and scope of the invention.

What is claimed is:

1. The powder rolling apparatus comprising a hopper, said hopper including bafile means, an orifice width adjustment plate and a pivoted lip, said orifice width adjustment plate and pivoted lip forming a rectangular metering orifice, the pivoted lip of said hopper being vibrated by controllable electronic driver means, two rollers, the rectangular metering orifice being above the said two rollers so that powder falling from the orifice will collect in the nip between the two rollers, at least one of said rollers being stepped, side retention plates placed in the recess caused by said step, the stepped surface being constantly roughened while rotating by abrasive means.

2. The powder rolling apparatus of claim 1 wherein said abrasive means impart grooves to at least one of said rollers, said grooves being of no greater thickness than about 0.3% of the as-rolled thickness of the strip.

3. The powder rolling apparatus of claim 1 wherein knife edges are inserted between the rectangular metering orifice and the rollers so that a portion of the powder stream is diverted from the roll gap.

References Cited by the Examiner UNITED STATES PATENTS 2,114,072 4/ 1938 Cleveland 29l2l 2,917,821 12/1959 Fritsch 29-4205 2,994,917 8/1961 Fritsch 8-9 FOREIGN PATENTS 245,560 6/ 1961 Australia.

WILLIAM J. STEPHENSON, Primary Examiner. 

1. THE POWDER ROLLING APPARATUS COMPRISING A HOPPER, SAID HOPPER INCLUDING BAFFLE MEANS, AN ORIFICE WIDTH ADJUSTMENT PLATE AND A PIVOTED LIP, SAID ORIFICE WIDTH ADJUSTMENT PLATE AND PIVOTED LIP FORMING A RECTANGULAR METERING ORIFIC, THE PIVOTED LIP OF SAID HOPPER BEING VIBRATED BY CONTROLLABLE ELECTRONIC DRIVER MEANS, TWO ROLLERS, THE RECTANGULAR METERING ORIFICE BEING ABOVE THE SAID TWO ROLLERS SO THAT POWDER FALLING FROM THE ORIFICE WILL COLLECT IN THE NIP BETWEEN THE TWO ROLLERS, AT LEAST ONE OF SAID ROLLERS BEING STEPPED, SIDE RETENTION PLATES PLACED IN THE RECESS CAUSED BY SAID STEP, THE STEPPED SURFACE BEING CONSTANTLY ROUGHENED WITH ROTATING BY ABRASIVE MEANS. 